This invention relates to improved metal surfaces for the first wall and limiter in a plasma device and more particularly to metal surfaces with reduced loss of metal by erosion and particularly by sputtering during exposure to the plasma. In a second aspect of the invention, it relates to a metallic substrate containing an amount of the surface metal and providing a self-sustaining source of the surface metal.
In a plasma device, plasma at an elevated temperature is magnetically confined within the first wall whose purpose is to limit the travel of particles escaping from the plasma. The limiter also serves to locate the plasma. During exposure to the hot plasma including bombardment by particles escaping from the plasma, surfaces of the first wall and limiter are subject to loss of material by erosion and particularly by sputtering. In the past, these surfaces have been constructed of metals such as stainless steel. Refractory materials such as ceramic and graphite coatings could also be useful. With ceramic materials, the extreme temperature changes may cause cracks to form and spallation to occur and therefore limit the effectiveness of the surface. Also, ceramic materials have limited value for removal of heat from the first wall and limiter and must therefore be applied as thin coatings over a metallic substrate. The failure of such coatings and clad layers could represent a serious materials problem for the development of plasma devices. With metal surfaces, the metal may be selected for its structural strength, resistance to corrosion, and ease of fabrication. However, metallic surfaces limit the effectiveness of the plasma device because they generate substantial amounts of sputtered neutral atoms which are not retained on the surface and contaminate the plasma. In addition, some metals selected for stuctural properties have a high atomic number and the presence of their atoms in the plasma reduces the energy available for the thermonuclear reactions.
It has been suggested that the alkali and alkaline earth metals could be useful as components of the first wall and limiter. Since these metals are electropositive, they are potentially capable of providing a high secondary ion yield. However, these metals in the pure state do not possess the desired structural, fabricating, physical and chemical properties. In addition, a significant portion of the metal spattered from pure alkali metal surfaces in practice escapes as neutral atoms and not as secondary ions.
As metal is lost from the surface, the useful life for operating of the plasma device is reduced because a new first wall or limiter or new surfaces for these metallic members must be installed. Accordingly, improvements in the first wall and limiter of the plasma devices are desired.
One object of the invention is a first wall or limiter for a plasma device with a surface exhibiting reduced loss of metal by erosion and particularly by sputtering.
A second object of the invention is a first wall or limiter for a plasma device with a surface exhibiting an improved secondary ion/neutral ratio.
Another object of the invention is a first wall or limiter with means for replenishing metal on the surface exposed to plasma in a self-sustaining manner.
Yet another object is a first wall or limiter with a surface coating of a low atomic number.
An additional object of the invention is a first wall or limiter with reduced loss of structural metal.
These and other objects will become apparent from the detailed description below.